1. Field of the Invention
The present invention relates generally to an alternate source of energy. More particularly the present invention relates to a three-vaned, drag-type wind turbine rotor for converting kinetic energy in the wind to shaft power.
2. Background Art
Wind turbines have a long and varied history. Windmills in The Netherlands were first used for grist, and later converted to raising water above sea level for land reclamation. In the late 19th and 20th centuries, wind power was commonly harnessed across the prairie and plains states in the U.S. for pumping water from wells. Also in the early part of the 20th century, wind was utilized for converting kinetic energy to electrical energy.
The last quarter of the 20th century saw a marked increase in interest in converting wind energy to shaft power. Many units from that era were horizontal shaft wind turbines using airfoils of various types. Drawbacks of such an arrangement are the need to have the power unit (generator, air compressor, etc.) on top of the tower with the airfoils, or the need for gearing to transfer the power toward the ground.
Efforts have been made toward improving vertical-shaft wind turbines as well. The Darius rotor utilizes airfoils in a fashion quite different than the horizontal shaft units. However, the Darius rotor is not self-starting, so a starting scheme is required.
The Savonius rotor is a self-starting, low-speed, drag-type wind turbine rotor. However, in its traditional form (see FIG. 1), the Savonius rotor is known to exhibit low efficiencies. It is a type of impulse turbine, as opposed to the reaction turbines having horizontal axes and the Darius rotor. Rotation of the Savonius rotor is effected through momentum transfer from the air. The momentum of the air changes as its path is curved by the vanes of the Savonius rotor. Momentum exchange occurs on entrance to the vanes and on exit from the vanes. The change in momentum with time results in forces that tend to turn the Savonius rotor on its axis of rotation.
A modification to the Savonius rotor of FIG. 1 was disclosed in U.S. Pat. No. 5,494,407. The blades of this invention have been altered from half-circles in cross-section as seen in FIG. 1 to the shape shown in FIG. 2, having a linear portion nearer the axis of rotation and a curved portion, which is substantially an arc of a circle tangent to the linear portion and tangent to the circle defining the rotor diameter.
Another modification to the Savonius rotor of FIG. 1 was disclosed in U.S. Pat. No. 6,283,711 wherein an additional, outer vane, is pivotally attached to the original, semicylindrical blade at the latter's leading edge.
A novel modification to the traditional Savonius rotor is shown in FIG. 3 wherein the vanes are reduced in size away from a vertical center such that they reach apexes at the top and bottom of the unit. Such a wind turbine can be made of light fabric material.
A modified Savonius rotor is disclosed in U.S. Pat. No. 7,008,171, which is herein incorporated by reference. In this modification of the Savonius rotor, illustrated in FIG. 4, the two vanes 410 form a single “S” shaped vane and channels 420 are provided, preferably along a lower edge of the vane 410 and extending at least halfway up the vane 410. Through the channel 420 is a flow path for the air to pass through the vane 410 from the convex portion of the vane 410 to the freestream. The exhaust channel 420 transitions from the vane 410 in a shape roughly similar to a cylinder diverging from the vane.
In all of the above prior art, two vanes are disclosed for each section of the respective drag-type, wind turbine rotor. Multiple sections may be stacked to increase power and enhance starting.
A three-vaned Savonius wind turbine rotor is disclosed in U.S. Pat. No. 7,220,107 and briefly illustrated in FIG. 5. The airflow through the vanes 510 of this device has the same characteristics as a two-vane Savonius rotor. As seen in FIG. 5, the incoming airflow impinges on a first vane 510, after which it is directed to a second vane 520 only.
The Savonius rotor and the rotor of the instant invention are referred to as “drag-type” wind turbines. In fact, each of the vanes is a drag-type device insofar as drag on the vanes is the force providing the torque to rotate the wind turbine. Drag is defined as the force imparted to an object by a fluid (the object moving relative to the fluid) in the direction of the relative motion of the object relative to the fluid. Drag is a subject of undergraduate fluid mechanics, and is well understood by those of ordinary skill in this art. Drag is discussed in virtually all undergraduate fluid mechanics texts, such as Fundamentals of Fluid Mechanics 5th ed. by Munson, Young, and Okiishi; John Wiley and Sons; 2005; ISBN 978-0-471-67582-2, which is hereby incorporated in its entirety by reference.
The drag due to relative fluid motion toward a concave surface is generally greater than that of relative fluid motion toward a convex surface. Hence, the force of the wind on the concave surface of a Savonius rotor is greater than the same wind's force on the convex surface of the same Savonius rotor. Therefore, the net force resulting in the torque that turns the rotor against its load is due to drag.
There is a need, therefore, for a drag-type wind turbine having three vanes, each of the vanes providing a flow path wherein, when the air leaves a given vane, the air is divided up into two paths and impinges on both the other vanes before being exhausted.